1. Field of the Invention
The present invention relates to an electronic ballast circuit for starting and powering a vapor discharge lamp having a phosphor layer for emitting light, commonly known as a fluorescent lamp, in which filaments constituting opposite electrodes of the lamp are preheated for a predetermined period of delay after which a high frequency operating voltage is applied across opposite electrodes of the lamp. The invention also relates to a method of starting a fluorescent lamp to improve the life of the lamp.
2. Description of the Related Art
Circuits for powering metal vapor fluorescent lamps which start the lamps in a manner in which filaments are first preheated are known from U.S. Pat. Nos. 4,256,992 and 4,928,039. Therein, as is common, lamps operate on a voltage of alternating polarity (usually more than 30 KHz) such that filaments constituting electrodes at opposite ends of the lamp alternately serve as cathodes in each cycle. These circuits cause the filaments to be conditioned by being preheated to incandescence during a predetermined period of about two seconds prior to turning on the lamp. In the circuits described, the operating voltage does not go through a sharp on to off transition since at least a separate inverter is started to produce the operating voltage, producing oscillations which take numerous cycles to build and stabilize.
The filaments constituting the lamp's electrodes are generally made from coiled tungsten wire and are coated with a material for enhancing their thermionic emission of electrons. During lamp operation, and particularly during turn on, tungsten and emitter material can evaporate or sputter from the electrodes. The amount of such evaporation and sputtering determines the lamp's remaining starting life (the estimated number of on-off starts until the lamp fails). Lamps of relatively poor quality, particularly in lamp applications with electronic ballasts employing so-called "rapid start" circuits, are particularly susceptible to the early appearance of end blackening of the lamp sidewall, which evidence that the filaments have already deteriorated due to excessive sputtering of filament materials and that the remaining starting life of the lamp is quite limited. In such lamps of poor quality end blackening may appear after as little as several hundred on-off starts.